Implement Some Tokio Tutorials

Tutorial/Future.rs

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+#![allow(non_snake_case)]
+
+use futures::future::poll_fn;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::time::{Duration, Instant};
+
+struct Delay {
+    when: Instant,
+}
+
+impl Future for Delay {
+    type Output = &'static str;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<&'static str> {
+        if Instant::now() >= self.when {
+            println!("Hello world");
+            Poll::Ready("done")
+        } else {
+            // Ignore this line for now.
+            cx.waker().wake_by_ref();
+            Poll::Pending
+        }
+    }
+}
+
+#[tokio::main]
+async fn main() {
+    let when = Instant::now() + Duration::from_millis(10);
+    let mut delay = Some(Delay { when });
+
+    poll_fn(move |cx| {
+        let mut delay = delay.take().unwrap();
+        let res = Pin::new(&mut delay).poll(cx);
+        assert!(res.is_pending());
+        tokio::spawn(async move {
+            delay.await;
+        });
+
+        Poll::Ready(())
+    })
+    .await;
+
+    /*
+    let when = Instant::now() + Duration::from_millis(1000);
+    let future = Delay { when };
+
+    let out = future.await;
+    assert_eq!(out, "done");
+
+    let when = Instant::now() + Duration::from_millis(10);
+    let mut delay = Some(Delay { when });
+    */
+}